This invention relates to linear position measurement systems, and in particular to a digital absolute position measurement system. Linear position measurement systems include a linear member and a second member which is movable along the length of the linear member. Such systems are widely used, two common applications being in positioning automated machinery and in measuring liquid level.
Numerous position measurement systems are presently used in automated equipment systems. Analog systems of considerable sophistication are known; such a system is set out in Moseley, U.S. Pat. No. 3,075,132 (1963), for example. However, the well-known advantages of digital systems have made them preferred. The known digital systems, some of their present uses, and some of their drawbacks are described in Beach et al. U.S. Pat. No. 3,584,284 (1971), and in Waller, U.S. Pat. 3,473,098 (1969). As set out in these patents, a first type of digital measurement system provides an absolute address for each location on the fixed linear member and the movable member reads each address as it passes. A second type provides unlabelled increments along the length of the fixed linear member, the movable member produces an output pulse for each increment it passes, and position is measured in terms of the number of output pulses produced in a given direction relative to a reference point. In both types, the transducer for producing readout signals is frequently an optical transducer consisting of one or more light sources and a corresponding number of photosensitive devices, the optical coupling between which is modulated by the indicia (e.g., reflective or transparent bands) carried by the linear member.
Some increment counting systems utilize active conductor portions in both the linear member and a closely spaced slide; the conductor portions are inductively coupled to produce digital increment signals representing traversal of an increment and analog signals representative of position within a particular increment. Such systems have been marketed under the registered trademark Inductosyn and are described, for example, in Tripp et al., U.S. Pat. No. 2,799,835 (1957), and Farrand, U.S. Pat. No. 3,202,948 (1965).
Liquid level measuring devices are known which include a vertically mounted linear member which extends into the liquid and a movable member in the form of a float slidably mounted on the linear member. Neither the optical linear position measuring systems nor the Inductosyn system, however, is very well adapted for use in liquid level measuring devices. The optical transducer is not compatible with immersion in many liquids, nor is the coded linear scale, and both are sensitive to contamination. The difficulties in raising (or lowering) the float to a reference position periodically or after a power outage severely limits the usefulness of any increment counting system. Finally, all of the systems previously described require electrical connections to the movable member (float). The weight and elasticity of the power line must be compensated for as the float rises and falls, and in an Inductosyn system inductive coupling of this line must be minimized.
The linear measurement systems which have been used for liquid level measurement have almost all been analog systems, generally of the variable resistance type. Some of the better systems have used reed switches, activated by a magnet in the float, as shunts to provide a stepped output, as for example in Levins U.S. Pat. No. 3,200,645 (1965). These systems suffer the drawbacks common to analog systems, and the reed switch systems have the further problems and expense of accurately positioning a reed switch for each measurable position of the float's travel.
One absolute linear position measurement system used in a liquid level measuring device is disclosed in French Patent No. 1,419,367 (1965) in which a separate float and switch are provided at each discrete position along the linear member and all of the switches are connected in parallel so that each position is indicated by the closing of an individual circuit. Another absolute system, described in Ordorica et al., U.S. Pat. No. 3,154,946 (1964), uses a separate transformer at each discrete position. The transformers are sequentially activated to determine which one is coupled through a magnetic float and therefore produces a higher amplitude output pulse. Such systems are obviously subject to severe practical limitations.
The failure of the prior art to provide an acceptable linear position measurement system for liquid level measurement is attested by the number of indirect level measurement systems which have been proposed and actually used. One widely used system eliminates the linear member, and the position of the float is determined by the length of a cable attached to the float. The cable is played out over a pulley above the liquid, and the excess cable is wound and stored on a drum. The length of cable played out is determined by digital or analog sensing devices associated with the cable, the pulley or the drum. Such a system is shown, for example, in Wright, U.S. Pat. No. 3,069,656 (1962). This type of system has a number of moving parts, requires careful compensation for a number of variables, and is subject to inaccuracies caused by wear of the moving parts, stretching of the cable, and the like. Even less direct systems involve the use of ranging apparatus for reflecting signals off the surface of the liquid, the placing of capacitance probes at intervals along the side of the liquid-containing vessel, the weighing of the entire vessel by means of load cells under the vessel, or determining the vessel's contents by means of pressure transducers at the bottom of the vessel. All of these devices have limited range, have limited accuracy, require careful field installation or compensation for the particular liquid being measured, and are subject to long term drift.